The invention relates to electronic printers, and more particularly to an improved image halftoning system for printers.
Conventional xerographic copiers do not render faithful or pleasing copies of continuous tone originals. The usual discharge characteristic of the photoconductor and solid area developability of the xerographic development system combine to yield a Tone Reproduction Curve (TRC) with a steep slope and a narrow range. The result is a copy with washed out highlights and overdeveloped shadows. The standard technique used to improve the gray scale reproduction is to expose the photoconductor through a transmissive screen to produce a fine, spatially modulated voltage pattern on the photoconductor. Development of this modulated pattern yields a Tone Reproduction Curve with a lower slope and an extended range of input gray scale that produces a corresponding change in the output. The result is a more faithful and pleasing copy.
A similar effect is found in the halftoning process employed by xerographic based printers in digital printing of pictures. There the image is formed of a textured pattern of black and white spots, which gives the impression of a gray when viewed at normal reading distance. If the halftone frequency and number of distinguishable halftone steps are both sufficiently high, the printed picture will be pleasing to the eye. Halftone methods employed in binary xerographic printers have traditionally been binary, that is, the laser writes with only two laser intensity levels: on and off.
When assessing the quality of a binary xerographic printer, two measures are important: the halftone frequency (i.e. number of halftone cells per linear inch), and the number of distinguishable gray steps. To produce a copy of a picture with a just acceptable degree of halftone graininess requires at least 65 halftone cells per inch measured along a diagonal of the page. Good quality halftones require about 100 cells/inch, while high quality magazines typically use 150 cells/inch or higher. The needed number of distinct gray steps in the pictorial copy depends upon the eye's ability to distinguish closely spaced grays. A rule of thumb in the printing industry is that an acceptable picture should contain about 65 gray steps. For good quality, 100 or more steps are desired.
However, in a binary printer, the maximum number of output gray steps is limited to the number of pixels per halftone cell (p), plus 1. Thus for a typical 8 by 4 rectangular halftone cell, p+1=33 output gray steps. Higher halftone frequencies have fewer pixels per cell and therefore produce fewer gray steps. This is the fundamental limitation of binary printers.
In the prior art, U.S. Pat. No. 4,078,929 to Gundlach discloses a two-color system in which a three level charge pattern is produced, for example, by modulating a laser beam to permit selective image developing by two color developers using positively and negatively charged toner particles. And, Fundamentals of Digital Copiers (Canon, Oct. 1984), discusses halftone density processing in which black and white pixels are arranged in cells in different combinations to provide a series of gray level steps. Additionally, this publication describes a trinary coding system in which the individual cell pixels, in addition to being either all white or all black, can also be composed of one-half white and one-half black. Another U.S. Pat. No. 4,434,431 to Ohkubo et al, discloses a multi-level printer having a control circuit for controlling a laser diode to vary the intensity of the beam output by the laser in response to multi-level image signals, while U.S. Pat. No. 4,651,287 to Tsao, discloses a system in which either the dot size or dot intensity of a laser beam is controlled in accordance with a preset algorithm in order to improve print accuracy.
U.S. Pat. No. 4,714,964 to Sasaki discloses a method of using two or more kinds of sizes of dots and two or more coloring agents having different concentrations for reproducing of continuous tone for use on an Ink Jet printer. U.S. Pat. No. 4,692,773 to Saito et al. discloses an image forming method having different concentrations of ink and size to reproduce proper gradation level. U.S. Pat. No. 4,491,875 to Kawamura discloses a method of constituting micro pixels with luminosity that can be designated according to predetermined gradation.
In addition, U.S. Ser. No. 07/197,051 filed May 20, 1988 (now U.S. Pat. No. 4,868,587 issued Sep. 19, 1989) and assigned to the same assignee as the present invention, discloses a multilevel laser to expose pixels at more than one level of exposure including an array of halftone cells having a plurality of pixels, the pixels having either a white, black, or gray reflectance level, the ratio of pixels having the gray reflectance level to the pixels having the black reflectance level or the ratio of pixels having the gray reflectance level to the pixels having the white reflectance level in each of the cells being chosen to provide a substantially uniformly changing series of average reflectance levels ranging from white to black in the array of halftone cells.
It is known to use three or four alternatives rather than a simple binary designation to identify pixel grey levels as disclosed in the above pending U.S. patent application. For example, it is known to modulate a laser beam to provide a black pixel, a white pixel, and two intermediate grey levels. Such a system, however, often does not provide the maximum number of possible grey level cell averages for halftone reproductions. Often, also, only a select number of all possible grey level cells is needed for halftone reproductions discernible to the human eye. Therefore, in such systems, even if a large number of grey level averages are provided, such systems do not provide a reliable method to select the most appropriate number of possible cell grey level averages in the displayed or printed image. The difficulty is how to provide the maximum number of grey level averages for the reproduced image cells, as well as to be able to select the appropriate grey level averages to be used in the reproduction.
It is an object of the present invention, therefore, to provide an easy method to provide the maximum number of average grey level cells for reproducing an image. Another object of the present invention is to provide a simple and adaptable method for selecting a suitable set of cell grey level averages from a much larger population of grey level averages to enhance the halftone reproduction. Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.